Updating database columns and merging serialized array data.

When importing, how can one keep an existing data in one column, while adding some data to it?

Example:
I import users' lists into the DB.
For already-existing users, I only want to update few columns, especialy the 'data' column in the 'users' table. But since some data already exists in that column, I want to keep it, and only ADD the new data to it.

How is that done?
Until now the only solution I found for not overriding the existing data was to unset the field in the entity definition, but that of course, is prohibiting me from updating the field, which is what I want to do...

I'd appreciate a lot even only a direction...